1. Field of the Invention
The present invention relates generally to a display device, and more particularly to a projection type display device using a liquid crystal display panel.
2. Description of Related Background Art
In recent years, much attention has been paid to a projection type liquid crystal display device as a display device for displaying on a large screen. The projection type liquid crystal display device may include several types, one of which is a type that forms a transmissive/non-transmissive two-dimensional pattern on a liquid crystal panel by use of laser beams, and a type that electrically forms the transmissive/non-transmissive two-dimensional pattern by use of thin-film transistors, etc. as a switch element. Particularly, the liquid crystal display device using the liquid crystal panel for electrically forming the display pattern is capable of displaying motion pictures and is therefore expected as the display device for a large screen TV.
Putting an emphasis on especially color display devices among the projection type liquid crystal display devices each using the display panel operating as a light bulb like the liquid crystal panel does, those devices are classified roughly into a 3-plate type liquid crystal display device and a single plate type liquid crystal display device.
The 3-plate type liquid crystal display device is constructed to form a color image by separating the light beams from a light source into three primary colors through a dichroic mirror, etc., subsequently producing a light intensity distribution of the separated light beams through the liquid crystal panel and synthesizing them by again using the dichroic mirror. On the other hand, the single plate type liquid crystal display device is constructed to actualize a color display by use of a single display panel as illustrated in FIG. 42, wherein one display panel may suffice, and therefore it can be considered that this might facilitate a reduction in costs.
Now, in the projection type display device using a matrix transmissive control type display panel typified by the liquid crystal panel, a transmissive area actually contributing to the display is normally considerably small for an area occupied by the pixel defined as one unit of the matrix forming the image. This is because a large proportion of wirings and interconnections for providing a drive voltage or current needed for operating the pixels and of the thin-film transistors defined as the switch element are opaque, and because there is no alternative but to restrict an effective transmissive area for the reason that the transmissivity is hard to precisely control at a boundary between the pixels.
A portion of the transmissive area contributing to the display, i.e., an opening between the pixels, may be termed a "pixel aperture".
Normally, the irradiation light beams upon the display panel are incident thereon with a substantially uniform intensity on the pixel unit,; however, the light beams incident on the areas excluding the pixel apertures are shut off.
Such being the case, there is known a technology of enhancing a light utilizing efficiency of the projection type display device wherein optical image forming elements corresponding to the dispositions of the pixels are provided on the incidence side of the irradiation light beams upon the pixel apertures to thereby form a luminance distribution correlated to the pixel aperture, and a rate of the transmitted light beams is thus increased. Normally, the image forming elements arranged on the incidence side of the pixels involve the use of minute lenses arranged in an array, which is referred to as a micro lens array.
FIG. 40 is an explanatory view illustrating an optical construction of the 3-plate type projection display device using the micro lens array.
A parabolic surface reflector 4011 formed with an aperture on the light irradiating side is disposed around a metal halide lamp 4010 serving as a light source for emitting the irradiation light beams. A filter 4012 for cutting off infrared rays and ultraviolet rays is disposed on the side of the aperture of the parabolic surface reflector 4011.
First and second dichroic mirrors 4013, 4014 for separating the irradiation light beams into three primary colors are disposed at an angle of approximately 45 degrees to the filter 4012 on the side of irradiating the irradiation light beams through the filter 4012. A first mirror 4015 for reflecting red light beams reflected by the first dichroic mirror 4013 is disposed substantially in parallel to the first dichroic mirror 4013. A first liquid crystal module driven by electric signals relative to a red color image, i.e., by red color image signals, is disposed in a position irradiated with the red light beams reflected by the first mirror 4015. The first liquid crystal module is constructed of an incidence-side polarizing plate 4031, a micro lens array 4032, a liquid crystal panel 4033 driven by the red color image signals, an exit-side polarizing plate 4034, and a field lens 4035, which are arranged in this sequence from the incidence side of the image signal to the exit side thereof.
Green light beams of the irradiation light beams penetrating the first dichroic mirror 4013 are reflected by the second dichroic mirror 4014, while blue light beams penetrate the second dichroic mirror 4014.
A second liquid crystal module including a liquid crystal panel 4043 driven by electric signals relative to a green color image, viz., by green color image signals, is disposed in a position irradiated with the green light beams. A third dichroic mirror 4017 is disposed at an angle of approximately 45 degrees to the liquid crystal panels 4033, 4043 of the first and second liquid crystal modules on the exit side of the first and second liquid crystal modules. The third dichroic mirror 4017 transmits the light beams emerging from the first liquid crystal module but reflects the light beams emerging from the second liquid crystal module.
A third liquid crystal module including a liquid crystal panel 4055 driven by blue color image signals is disposed in a position irradiated with the blue light beams penetrating the second dichroic mirror 4014. A second mirror 4016 is disposed at an angle of approximately 45 degrees to the liquid crystal panel 4053 on the exit side of the third liquid crystal module.
A fourth dichroic mirror 4018 is disposed substantially in parallel to the third dichroic mirror 4017 and the second mirror 4016 in a position irradiated with the blue light beams from the second mirror 4016 as well as with the light beams from the third dichroic mirror 4017. The fourth dichroic mirror 4018 transmits the light beams coming from the third dichroic mirror 4017 but reflects the light beams coming from the second mirror 4016, wherein the three primary color images emerging from the respective liquid crystal modules are synthesized. The synthesized images are projected on a screen 4020 via a projection lens 4019.
Next, an operation of the 3-plate type projection display device using the micro lens array shown in FIG. 40 is described.
The light beams emitted from the metal halide lamp 4010 are substantially collimated by the parabolic surface reflector 4011, and turn out to be white light beams, unnecessary invisible beams of which are eliminated by the filter 4012 for cutting off the infrared rays and the ultraviolet rays. The white light beams are then incident upon the first dichroic mirror 4013. The first dichroic mirror 4013 has such a characteristic as to reflect only the red light beams, and the reflected red light beams are further reflected by the first mirror 4015 and incident upon the incidence-side polarizing plate 4031 of the first liquid crystal module. The red light beams turned out by the red polarized light beams, by the incidence-side polarizing plate 4031, are incident on the micro lens array 4032 and then incident upon the liquid crystal panel 4033 drive by the red color image signals. The pixels of the liquid crystal panel 4033 operate in a 90-degree twisted nematic mode, wherein the polarized light beams incident when in the brightest display are optically rotated through 90 degrees and allowed to penetrate without the optical rotation when in a dark display.
The micro lenses of the micro lens array 4032 are disposed in the position corresponding to the pixel apertures of the liquid crystal panel 4033, and function to converge the incident light beams at the pixel apertures.
FIG. 41 is an explanatory view schematically showing how the micro lenses of the micro lens array converge the incident light beams at the pixel apertures of the liquid crystal panel.
The light beams incident on the micro lenses of the micro lens array 4101 penetrate a light incidence-side substrate 4102, then converge at the apertures between the pixels 4103 and emerge from through a light exit-side substrate 4104.
Referring back to FIG. 40, the red light beams penetrating the pixel apertures of the liquid crystal panel 4033 at a high efficiency due to the function of the micro lens array 4032 become light intensity images because of the exit-side polarizing plate 4034 functioning as a filter for changing the transmissivity in accordance with a state of the optical rotation by the liquid crystal panel 4033. The images are then projected in enlargement on a screen 4020 by a field lens 4035 and a projection lens 4019.
The third and fourth dichroic mirrors 4017, 4018 are provided between the field lens 4035 and the projection lens 4019. However, the third dichroic mirror 4017 has such a characteristic as to transmit the red light beams but reflect the green light beams, while the fourth dichroic mirror 4018 has such a characteristic as to transmit the red and green light beams and reflect only the blue light beams, with result that the red light beams are not influenced.
On the other hand, the green and blue light beams penetrating the first dichroic mirror 4013 are separated by the second dichroic mirror 4014. The thus-separated light beams turn out to be green and blue images with the light intensities controlled, as in the case of the red light beams, by the liquid crystal panel 4043 driven by the green color image signals and by the liquid crystal panel 4053 driven by the blue color image signals. These green and blue color images are synthesized with the red color image by the third and fourth dichroic mirrors 4017, 4018 and the second mirror 4016 as well, and then projected in enlargement.
Now, there must be a variety of display devices for actualizing the color display by only one display panel. The typical display device may, however, be a display device of such a type as to provide color filters corresponding to respective aggregations of the pixels driven by the three primary color intensity signals on the display panel, and display device of such a type as to actualize the color display by disposing the image forming element array just anterior to the display panel, i.e., on the light incidence side, irradiating the light beams of the three primary colors at different angles, and making incident the light beams of the color corresponding to the pixels driven by each of the three primary color intensity signals.
When these two types of display devices are compared, the display device using the color filters causes a loss of the optical signals because of the color filters absorbing or reflecting two colors among the three colors. The display device using the image forming element array, however, causes no such a loss and is therefore expected as a device capable of attaining the display brighter than by the display device using the color filters.
FIG. 42 is an explanatory view showing an optical construction of a single plate type color display device using the image forming element array. In the constructive example illustrated in FIG. 42, the micro lens array is used as the image forming element array, and there are employed a plurality of dichroic mirrors arranged at different angles to separate the light beams falling upon the image forming elements into three colors according to the angles.
An elliptical reflector 4211 formed with an aperture on the light irradiating side is disposed around a metal halide lamp 4210 serving as a light source for emitting the irradiation light beams. A conical lens 4212 for collimating the light beams is provided on the side of the aperture of the elliptical reflector 4211. A stop 4213 for controlling a light angle distribution is so disposed on the light exit side of the conical lens 4212 as to cover a peripheral edge of the conical lens 4212.
Disposed in sequence on the light exit side of the conical lens 4212 and the stop 4213 are a filter 4214 for cutting off the infrared rays and the ultraviolet rays, a condenser lens 4215, and a color separation mirror unit 4216 constructed of three pieces of dichroic mirrors. The color separation mirror unit 4216 includes the three dichroic mirrors arranged making minute angles to each other to separate the incident light beams into three primary color light beams at reflection angles deferent from each other.
A liquid crystal module consisting of an incidence-side polarizing plate 4217, a micro lens array 4218, a liquid crystal panel 4219 and an exit-side polarizing plate 4220 that are arranged in this sequence is disposed in a position in which to reflect the light beams separated into the three primary colors by the color separation mirror unit 4216. A field lens 4221 and a projection lens 4222 are dispose on the exit-side of the liquid crystal module. Further, according to the actual construction, there is used the screen on which the image projected through the projection lens 4222 is formed. This constructive element is omitted in illustration on the drawings. What has been so far described is the optical construction of the single plate type color display device using the image forming element array.
The operation of the single plate type color display device using the image forming element array shown in FIG. 42 will hereinafter be explained.
The light beams from the metal halide lamp 4210 are converged at a position just before the light-incidence side surface of the conical lens 4212 by a rotary elliptical reflector 4211. Conical lens 4212 collimates the light beams more easily by concentrating exit angles of the converged light beams on the optical axis side of the exit light beams. The reason why the stop 4213 restricts the exit angle of the light beams emerging from the conical lens 4212 is that the light angle distribution obtained on the exit side of the condenser lens 4215 is determined based on the area of the light exit portion of the stop 4213 and the focal length of the condenser lens 4215, and hence a predetermined angle distribution which will be mentioned later on is to be obtained.
The light beams emerging from the stop 4213, after unnecessary invisible light beams thereof have been eliminated by the filter 4214 for cutting off the infrared rays and the ultraviolet rays, are incident upon the condenser lens 4215.
The condenser lens 4215, if manufactured of a spherical lens of a normal glass, becomes thick with a large spherical aberration, and therefore what is used is a Fresnel lens that is reduced in weight and in aberration.
The light beams substantially collimated by the condenser lens 4215 are incident on the color separation mirror unit 4216 composed of three pieces of dichroic mirrors. The characteristics of the three dichroic mirrors constituting the color separation mirror unit 4216 are as follows. Assuming that these three dichroic mirrors are designated respectively by 4216R, 4216G, 4216B, the dichroic mirror 4216R reflects only the red light beams of the visible light beams, similarly the dichroic mirror 4216G reflects only the green light beams, and the dichroic mirror 4216B reflects only the blue light beams.
FIG. 43 is a graph showing one example of the characteristics of the three dichroic mirrors constituting the color separation mirror unit 4216.
Curves B, G, R in the graph indicate the characteristics of the reflectance with respect to the light wavelengths of the dichroic mirrors 4216R, 4216G, 4216B. That is, as described above, the dichroic mirror 4216R reflects only the red light beams,; similarly the dichroic mirror 4216G reflects only the green light beams, and the dichroic mirror 4216B reflects only the blue light beams.
FIG. 44 is an explanatory view schematically showing angles at which the three dichroic mirrors constituting the color separation mirror unit 4216 are disposed.
As explained above, the three dichroic mirrors 4216R, 4216G, 4216B of the color separation mirror unit 4216 are disposed making the minute angles each other to separate the incident light beams into those of the three primary colors having the reflecting angles different from each other. To be specific, the dichroic mirror 4216B centered among the three dichroic mirrors 4216R, 4216G, 4216B is disposed to make an angle of 45 degrees to the optical axis of the incident light beams, and on both sides thereof the dichroic mirrors 4216R, 4216G are disposed to make an angle of approximately 2.3 degrees to the dichroic mirror 4216B. The three dichroic mirrors 4216R, 4216G, 4216B are thus disposed, whereby the incident light beams are separated into optical signals of the three primary colors having different angles in the horizontal direction of the liquid crystal panel 4219.
As discussed above, the light beams angularly separated into the three primary colors are, after being polarized by the incidence-side polarizing plate 4216 of the liquid crystal module in FIG. 42, incident on the liquid crystal panel 4218 and the micro lens array 4217 as well.
FIG. 45 is an explanatory view schematically showing a relationship between the micro lens array and the liquid crystal panel. FIG. 45 shows some of cross sections of the micro lens array 4501 and of the liquid crystal panel as well as showing how the light beams are incident on one micro lens of the micro lens array 4501. The liquid crystal panel is constructed of a light incidence-side substrate 4502, a pixel unit 4503 and a light exit-side substrate 4504.
In three primary color light beams R,G,B, R represents the red light beam, G is the green light beam, and B is the blue light beam, which have been angularly separated, are incident upon one image forming element (the micro lens) of the micro lens array 4501 defined as the image forming element array. The three primary color light beams R, G, B are each separately incident upon the aperture formed for every color in the single pixel of the pixel unit 4503 on the liquid crystal panel through the light incidence-side substrate 4502 constituting the liquid crystal panel.
At this time, there is established the following relationship between the thickness t and the refractive index n of the light incidence-side substrate 4502, the pixel pitch Ppexl and the image forming element pitch Plens with respect to the angle difference .DELTA..theta.: ##EQU1##
The thus color-separated light beams pass respectively through the corresponding color apertures and emerge from the light exit-side substrate 4504. Then, the light beams penetrate the exit-side polarizing plate 4220 in FIG. 42, thereby forming color images. The color images are projected in enlargement on the screen through the field lens 4221 and the projection lens 4222, and the display on the large screen is thus attained.
Incidentally, one of the problems inherent in the projection type display device may be such that a non-uniform display tends to easily occur. A cause of this problem lies in a difficulty of reducing the irradiated light beams off the display unit of the liquid crystal panel when converging the light beams from the lamp as the light source on the liquid crystal panel, and a difficulty of brightening even the edge areas of the display unit by irradiating the light beams as in the case of the central area.
FIG. 46 is an explanatory view schematically showing an incident luminance upon the liquid crystal display panel of the projection type display device.
When using a normal light source, the luminance is gradually decreased as it approaches a circumferential area of the illumination. Therefore, if used to get only the portion having a high uniformity of the luminance applied to the display panel as shown in FIG. 46A, the efficiency declines with a large futility of the light of the circumferential area. On the other hand, as shown in FIG. 46B, when reducing the light beams leaking outside the display panel, the circumferential area of the display region gets very dark, resulting in an increase in ununiformity of illuminance or of luminance. Further, the fact that the display unit normally takes a square shape is one of the causes to make it hard to converge the light beams.
As a mechanism for obviating the problems in terms of the illumination efficiency and the ununiformity of luminance described above, an illumination device using a pair of lens array has been proposed and employed.
FIG. 47 is a view schematically illustrating a construction of the illumination device using the pair of lens arrays.
In this illumination device, the illumination light beams are incident upon a first lens array 4701 and split into fluxes of light beams, the number of which corresponds to the number of lenses of the first lens array 4701. The light beams emerging from the respective lenses of the first lens array 4701 are incident upon the corresponding lenses of the second lens array 4702, and a display panel 4703 such as a liquid crystal panel, etc. is irradiated with the light beams merging from the respective lenses of the second lens array 4702.
A configuration of each lens of the first lens array 4701 is set analogous to the configuration of the display surface of the liquid crystal panel 4703. Then, a focal length of the second lens array 4702 is set so that the configuration of each lens of the first lens array 4701 is projected in substantially the same size as the display panel 4703, on the surface of the display panel 4703. It is therefore feasible to attain the high-efficiency illumination uniformly on the entire liquid crystal display surface.
FIG. 48 shows an example of the display device including the illumination device using the two lens arrays.
The light beams from a lamp 4801 are converged at a first lens array 4804a through an infrared/ultraviolet rays cut filter. The first lens array 4804a splits the light beams into a configuration analogous to the display panel and converges the light beams at a second lens array 4804b. The second lens array 4804b projects the light beams emerging from the individual lenses of the first lens array 4804a upon a display panel 4807. A condenser lens 4805 and a polarizing plate 4806 are disposed in front of the display panel 4807, whereby the light beams from the second lens array 4804b are substantially collimated and polarized, and then incident on the liquid crystal panel 4807. The light beams, a polarized state of which is modulated by the liquid crystal panel 4807, are modulated in light intensity by the polarizing plate 4808 and thus converted. The thus converted light beams are then projected through a field lens 4809 and a projection lens 4810.
There arises, however, a problem inherent in the light source using the lens array, wherein it is a difficulty to apply it to the display panel of such a type as to use the image forming element array. The reason why so is that angles of the light beams incident on the display panel get discrete in a multiplicity of directions, resulting in an extremely large angle distribution of the light beams from the light source.
FIG. 49 is an explanatory diagram in which an angle distribution of the exit light beams in a normal illumination optical system is displayed on the plane having an azimuth angle .phi. and an inclined angle .theta. of the incidence angle upon the display panel. FIG. 50 is an explanatory diagram in which an angle distribution of the exit light beams in the illumination device using a (3.times.5) lens array is displayed on the plane having the azimuth angle .phi. and the inclined angle .theta. of the incidence angle upon the display panel.
In the case of the normal illumination optical system shown in FIG. 49, there is shown a one-block distribution spreading with a state-of-vertical-incidence portion centered on the display panel. Contrastingly in the case of the illumination device using the lens array shown in FIG. 50, the light beams merging from the respective lenses of the lens array are angularly separated, and hence, if a light quantity remains unchanged, an entire distribution range is broadened in principle.
When the light beams are converged at the pixel aperture by the image forming element such as a micro lens, etc., disposed in the vicinity of the pixel on the display panel, a distance between the image forming element and the pixel aperture is restricted by a size of the angle distribution of the incidence light beams.
FIGS. 51A and 51B are explanatory views each schematically showing a maximum distance between the image forming element and the pixel aperture in the case of a small range of the incidence angle distribution of the incident light beams and in the case of a large range thereof.
When an incidence angle .theta.i shown in FIG. 51A is small, a distance t between the image forming element and the pixel aperture can be taken long, and therefore a divergence angle .theta.out through the image forming element also becomes small. In contrast with this, when the incidence angle .theta.i shown in FIG. 51B is large, it is required that the distance t between the image forming element and the pixel aperture be taken short. Accordingly, the divergence angle .theta.out through the image forming element also becomes large. That is, in the case of the illumination device using the lens array, the problem is that the image forming element and the pixel aperture are required to get close, and the divergence angle .theta.out becomes extremely large.
This problem similarly happens both in the case of aiming at an enhancement of the transmissivity through the display panel by the image forming element and in the case of using the image forming element for separating the colors according to the incidence angles. This makes it hard to apply the light source using the lens array to the projection type display device.
Furthermore, it is much harder to combine the light source using the lens array with the liquid crystal panel of the single plate type color display device in such a system as to separate the colors according to the incidence angles.
FIG. 52 is an explanatory diagram in which the angle distribution after being angularly separated into three colors is displayed on the flat surface in the case of the normal illumination optical system. FIG. 53 is an explanatory diagram in which the angle distribution after being angularly separated into three colors is displayed on the flat surface in the case of the illumination device using the lens array. When comparing those angle distributions in FIGS. 52 and 53, it can be understood that the illumination device using the lens array shown in FIG. 53 has the angle distribution much larger than in the case of the normal illumination optical system shown in FIG. 52.
As described above, the light beams wherein the divergence angle by the image forming element is further added to the above angle distribution are incident on the display panel and projected on a display screen through the projection lens. When the light beams having a large divergence angle are incident on the display panel, there might increase a possibility of reducing the light quantity due to an increase in the surface reflection or the like on the display panel. Further, if the display panel is a liquid crystal panel, there must be a nature (visual angle characteristic) in which the display characteristic varies depending on the light angle of the liquid crystal panel, and the use of the light beams having a large divergence angle exerts an adverse influence on the display performance.
Moreover, when projecting an image of the display panel upon the display screen through the projection lens in a state where the light beams having the large divergence angle are incident upon the display panel, there must be needed a projection lens having an extremely small F-number to project the light beams having the large divergence angle. The projection lens with the small F-number is generally expensive and heavy, and hard to reduce the distortion as well as being hard to enhance the resolution in terms of design. This might bring about increases in size and weight of the projection type display device, an increment in costs therefor and a decrease in performance thereof.
Moreover, the optical system using two sets of lens arrays has such a defect that a length of an optical path in the optical system gets elongate in addition to requiring the optical elements taking an intricate configuration, and a set size of the projection type display device increases. Also, the plurality of optical elements incorporated therein lead to an increase in the number of reflecting surfaces and in turn an optical loss, resulting in a decline of efficiency.